When manufacturing an aluminum electrolytic capacitor, an oxide film (dielectric film) is conventionally formed by chemically treating the surface of aluminum foil, the effective surface area of which has been increased via etching, thereby creating an anode. To form an oxide film having a withstand voltage of 200 V or higher for an anode used in such an aluminum electrolytic capacitor, the aluminum foil is boiled in purified water, then subjected to an aqueous borate or phosphate solution, and a thermal depolarization process of heating the aluminum foil is performed during the formation. It has also been proposed, when manufacturing an anode having a film withstand voltage of 200 V or higher, to perform the formation upon the aluminum foil using an aqueous solution of ammonium adipate, as in the case of low-voltage anodes, in order to improve capacitance (patent documents 1, 2).
Meanwhile, the use of a porous aluminum electrode constituted by an aluminum core and a porous layer formed by sintering powdered aluminum layered upon a surface thereof instead of an etched aluminum foil as an anode for an aluminum electrolytic capacitor has also been proposed. Such a porous aluminum electrode has the advantage of eliminating the need to perform etching using hydrochloric acid or the like (patent document 3). A porous aluminum electrode also has the advantages of allowing for the formation of a porous layer of sufficient thickness and creating a structure of intricately intertwined spaces, thereby allowing for increased capacitance.